The present invention relates to a method for detecting zero crossings in a power signal and, more particularly, to a method for detecting zero crossings in a rectified power signal for a three-phase circuit so as to enable detection of a ground fault condition in the circuit.
It is desirable to provide accurate and efficient detection of a ground fault condition that can occur in three-phase circuits, such as electric motors and the like. Accurate and efficient detection is desirable to avoid damage to the electric motor. For instance, if the ground fault condition is left uncorrected, any current which substantially exceeds the ampererating of the motor can result in motor temperatures being sufficiently high for damaging or degrading winding insulation. This degradation can eventually cause electrical shorts in the catastrophic failure. The power signal associated with such threephase circuits typically comprises three separate signal constituents, such as three separate sinusoids, each having an electrical phase which is displaced by 120.degree. with respect to one another. In general, when a ground fault occurs at least one of three signal constituents or sinusoids experiences a phase and/or magnitude shift. Thus, it will be appreciated that detection of a ground fault is equivalent to detecting such phase and/or magnitude shift. As will be recognized by those skilled in the art, the sum of three equal magnitude sinusoids offset from one another by 120.degree. in phase is equal to zero. Thus, the presence of a non-zero residual in the sum of the three sinusoids is generally indicative of the ground fault condition. However, since the available power signal is rectified as well as corrupted by third harmonics and other noise sources, monitoring for the ground fault condition through the sum of the three separate sinusoids requires substantially accurate detection of zero crossings in each of the three signal constituents or sinusoids that make up the rectified power signal. In essence, the zero crossing detection is required so that the power signal can be derectified and summed as explained above. Thus, there is a need for an accurate and efficient zero crossings detection method which can be readily performed on a rectified power signal and which is substantially immune to the presence of third harmonics and other noise sources.